1. Field of the Invention
The present invention relates to an apparatus and a method of measuring a terahertz wave, and more particularly, to an apparatus and a method of measuring a terahertz wave in a time domain. Hereinafter, such as an apparatus will be also referred to as a THz TDS (Time Domain Spectroscopy) apparatus.
2. Description of the Related Art
A terahertz wave is an electromagnetic wave with a frequency in an arbitrary frequency band within a range from 0.03 THz to 30 THz. This frequency range includes many frequencies or bands of frequency at which characteristic absorption occurs due to structures or states of substances such as biological molecules. This feature is usable to nondestructively analyze or identify a substance, and associated techniques have been developed. One example of a predicted application is a safety imaging technique usable instead of an X-ray imaging technique. Another example of a predicted application is a high-speed communication technique.
In the time domain, terahertz waves generally have a form of a pulse with a width of sub-pico seconds. It is generally difficult to acquire such a pulse in real time because of the slow response that current electronics have with respect to the speed of THz waves. To overcome such a difficulty, a conventional THz-TDS apparatus employs a sampling measurement technique using ultrashort pulse light with a pulse width on the order of femto seconds. The sampling of the terahertz wave is achieved by adjusting a difference between a time at which excitation light arrives at a generating unit that generates the terahertz wave and a time at which the excitation light arrives at a detection unit that detects the terahertz wave. For example the time difference can be provided by disposing a stage having a folded optical system (also referred to as an optical delay unit in the present description) in a propagation path of the excitation light and adjusting the total round-trip length of the excitation light in the folded optical system (see, for example, Japanese Patent Laid-Open No. 2008-20268). In many cases, the generating unit and/or the detection unit is realized using a photoconductive device including an antenna electrode pattern having small gaps formed on a semiconductor film.
In the THz-TDS apparatus, an increase in the measurement sensitivity can result in an increase in effects of a vibration of the stage of the optical delay unit. More specifically, the vibration of the stage used in the optical delay unit causes the optical axis of the excitation light to swing. This causes a change in the amount of light per unit area that strikes the small gaps of the photoconductive device. Thus, a vibration component is superimposed on the time-domain waveform of the terahertz wave reproduced by the apparatus. If such a time-domain waveform is subjected to a Fourier transform, then, as shown in FIG. 6, a resultant spectrum 623 of the terahertz wave detected by the detection unit includes a spurious component 624 due to the vibration of the optical delay unit. For example, when a vibration component of several hundred Hz is superimposed on a time-domain waveform of a terahertz wave, a spurious spectrum appears typically at 4 THz to 6 THz although the spurious spectrum varies depending on the configuration of the measurement system and/or the driving condition of the optical delay unit. Such a spurious spectrum can limit a measurement bandwidth of the measurement apparatus and can cause a reduction in analysis performance thereof. As can be seen from the above description, in the terahertz wave measurement apparatus, there is a need for suppression of effects of vibrations of an optical delay unit.